For years persons have been lost overboard and not been able to be located even when near their own ship. Although Emergency Position Indicator Radio Beacons (EPIRBs) have been widely used for decades, they have not been sufficiently small enough to be carried by individuals. Additionally, EPIRBs use radio frequency direction finding and therefore are not very accurate in pin-pointing an individual or even a small boat lost at sea. They are not even very effective for allowing the person's ship to locate the person who has fallen overboard. EPIRBs are typically used by the United States Coast Guard (USCG) service, which continually monitors for EPIRB emergency radio broadcasts and have the ability to determine the location of the EPIRB by Doppler signal processing from the satellite receivers. An EPIRB-determined location is only accurate to within several square miles and can be off as much as thirty square miles.
Other rescue locating devices include mirrors, strobes and lights, whistles flares and other items. These are certainly better than nothing but they are not ideal. In many instances, these devices must be operated by the overboard person who may quickly be overcome and incapacitated by the environment.
More recently, Global Positioning Systems (GPSs) have been used to locate lost persons. GPS systems determine position by receiving signals from a sub-set of the 24 U.S. GPS satellites that are in operation. The signals transmitted by each satellite include a time code, which is synchronized with the time codes transmitted by the other satellites. The GPS system calculates an earth-centered-earth-fixed (ECEF) position for the location where the signals are received, based on the time differences between the signals received from the satellites and the known location of the satellites. ECEF positioning uses a 3-axis coordinate system with the origin located at the center of the earth, and can be translated to a global coordinate-based system (i.e., latitude and longitude).
The GPS satellites are not in geo-synchronous orbits. The location of the satellites are determined from almanac and ephemeris data either downloaded from the ship's host system or from the satellites themselves. Almanac data are good for several weeks and are updated weekly. Ephemeris data are good for about 4 hours and are updated hourly. Almanac data consist of general information regarding all satellites in the constellation and atmospheric data for a determination of RF propagation delays. Almanacs have approximate orbital data parameters for all satellites. The typical ten-parameter almanac describes the satellite orbits over extended periods of time of up to several months and a set for all satellites is sent by each satellite over a period of 12.5 minutes minimally. Signal acquisition time on receiver start-up can be significantly aided by the availability of current almanac and ephemeris data. The approximate orbital data are used to preset the receiver with the approximate position and carrier Doppler frequency (i.e., the frequency shift carried by the rate of change in range to a moving satellite) of each satellite in the constellation. Ephemeris data consist of detailed orbital information for the specific observed satellite. It can take up to 15 minutes to initialize a GPS system if the almanac and ephemeris data are not available or not up to date.
GPS-based locator systems have required individuals to be equipped with a small GPS receiver and a radio to send their latitude and longitude coordinates over the radio. This requires the person to have both devices when lost, which is unusual for most recreational boating situations and typically only occurs with military pilots who are thus equipped. Another approach has been to carry a device that sends an RF emergency signal to the person's vessel or a nearby vessel when overboard, so that the crew on the vessel can broadcast an alert of the overboard situation and give at least the vessel's location with its on-board GPS navigation system when the emergency signal is received. This would provide rescue vessels with the position of the vessel from which the person has fallen overboard, but cannot provide the exact location of the person nor an update of the person's location adrift with the current. A person overboard in heavy sea often cannot be seen even one hundred feet away from a pending rescue vessel. If rescue is not accomplished quickly, that person can easily be lost. Larger ships and sailing vessels typically take time to turn around and retrace their course, so not having an exact location of the overboard person can greatly diminish the effectiveness of the rescue effort. In small sailing vessels often only one person is on watch; if that person falls overboard, it may be hours before it becomes known.